The present invention relates to color manipulation for digital image processing in the prepress industry.
Human perception of color changes described in terms of a CMY (cyan, magenta, yellow) color space is not intuitive. Traditional color manipulation requires the color changes to be specified in abstract CMY percentages. Some fundamental color manipulations, e.g. brightening, saturating and turning a "color wheel", cannot be defined in these terms.
Stated differently, when you change one or more of the colors, C, M, Y, and K, the resultant color change is not normally humanly predictable, since colors result which are unexpected, based on human experience.
As a result, it has required highly trained operators involved in the pre-press industry to know what color will result when various changes are effected in C, M, Y, and K.
In a first prior art system and method as shown in FIG. 1, an image scanner 1 outputs C.sub.orig, M.sub.orig, Y.sub.orig, and K.sub.orig color signal values, derived by scanning an original image, to a computer or hardware circuit 2 which performs a CMYK transformation to create new color values C.sub.new, M.sub.new, Y.sub.new, and K.sub.new. Here an operator inputs desired corrections to the image by inputting individual C, M, Y, and K changes. In other words, a specific change for C.sub.orig to C.sub.new is input independently of changes for the other three colors. The new color values C.sub.new, M.sub.new, Y.sub.new, and K.sub.new are then input to a color computer which takes into account the paper type on which the image is to be eventually printed. This is necessary since various paper types have various effects on the resulting color of inks when the inks are applied to the paper. The color computer also converts the C.sub.new, M.sub.new, Y.sub.new, and K.sub.new values to R, G, B, video signals and then inputs the color video signals to a well known color video display 4. As a result, the operator can see on the color video display 4 a true rendition of the colors as they will be printed on the paper, but after the corrections have been input. Thus, the operator can see precisely how the image will appear later in print.
A problem with this approach shown in FIG. 1 is that it requires a highly trained operator to know how the change input for one of the colors C, M, Y, and K will affect the overall result in terms of hue, saturation, and luminance. Thus, the prior art technique in FIG. 1 is laborious and time consuming and requires highly trained operators.
A second prior art approach is shown in FIG. 2.
As a result of attempts to introduce more intuitive color manipulation, HSL (Hue, Saturation, Luminance) changes are executed in three steps. In a first step, the original image from scanner 1 is translated by translation 5 from the printable CMY color space to the HSL (or equivalent) color space, which is unprintable but convenient for calculation. In a second step, the HSL changes are applied in an HSL transformation 6 according to a binary mask and are executed in a new color space. In a third step, the new image is translated by translation 8 back to the printable color space. EQU (CMY).sub.original .fwdarw.color space translation.fwdarw.[HSL(or equivalent)].sub.original EQU [HSL(or equivalent)].sub.original .fwdarw.HSL change.fwdarw.[HSL(or equivalent)].sub.new EQU [HSL(or equivalent)].sub.new .fwdarw.color space translation.fwdarw.(CMY).sub.new
In this approach shown in FIG. 2, it is difficult to: 1) accomplish quality translations between color spaces; 2) meet precision requirements; 3) decide how to treat unprintable colors; 4) achieve interactive performance; 5) avoid the undesirable effects of binary weighting; and 6) the new color values resulting from corrections, namely C.sub.new, M.sub.new, Y.sub.new, and K.sub.new may lie outside the color space and be impossible to print, even though the colors have been properly rendered on the video display.